big iron

noun

variants:
orBig Iron

Definition of big iron

computing

:large mainframe computers

… it will wither as most customers find the courage to shift to so-called client-server systems, which are networks of smaller computers. When that happens, other IBM businesses that depend on big iron will also decline.

—Stratford Sherman, Fortune, 3 Oct. 1994

Next month, several large companies will lead the charge back to big iron in an attempt to rescue unwieldy or stalled client-server projects.

—Martin J. Garvey, Information Week, 7 July 1997

A few years ago the big iron would almost certainly have been provided by the likes of IBM, Hewlett-Packard or Digital Equipment Corp.